1. Field of Invention
The present invention relates generally to a device for controlling pitch and roll motions of marine vehicles and, more particularly, to a dynamic control system for stabilizing pitch and roll motions, as well as for providing steering, using a single set of control surfaces, thus, minimizing impact on the ship design in terms of size, resistance, and cost.
2. Brief Description of Related Art
Pitching is one of the most damaging motions for a ship. With respect to Naval combatant vessels, ship pitching motion detrimentally affects combatant operations in many ways. Vertical motions and accelerations can reduce the effectiveness of bow-mounted sonars as well as towed arrays. Slamming of the bow dome causes local distortions and damage. Angular acceleration affects the performance of radars and weapons, and can cause misalignment through hull flexure. Motion-induced added resistance and keel slamming cause involuntary speed reductions and may necessitate voluntary speed reductions to reduce damage. Emergence of propellers and rudders in following seas result in propeller racing and loss of steerage. Keel and bow flare slamming cause structural vibrations that can damage the hull and equipment. Spray generation reduces visibility and increases topside icing in cold weather. Vertical accelerations affect crew performance through seasickness, loss of balance and injury. Accordingly, there is a present need to improve ship operability (e.g., percent of time the ship is operable to perform its combat mission) by reducing pitching motion. However, since the introduction of anti-roll fins in the 1940's, and rudder-roll stabilization in the 1970's, roll reduction serves as the primary means to improve operability.
Attempts at pitch stabilization began soon after the introduction of roll stabilization in the 1860's, with the majority of efforts being undertaken this century. However, all past attempts have either been unsuccessful due to undesirable peripheral effects such as vibration, or involve greatly increased complexities and costs in the ship design and operation. The requirements for pitch stabilization devices are somewhat different than those required for roll stabilization. The enormous moments generated in pitching, often over fifty times the roll moment, require that pitch stabilization devices generate equally large counteracting moments. For surface combatants, pitch periods are typically half those of roll periods (5-7 seconds for pitch as compared to 10-15 seconds for roll), so any anti-pitch device requires a quick reaction time. Moreover, pitch motions are excited over a much wider frequency range than roll motions, thus precluding stabilization methods that are only effective at certain frequencies. Consequently, such motion control devices as gyroscopes and tanks are not suitable as anti-pitch devices. Gyroscopic system would require massive flywheels to counter pitching moments. Passive internal tanks are generally highly tuned making them ineffective at off-design frequencies, whereas active tanks would require such a fast flow of immense amounts of water as to make them impractical. Consequently, all feasible attempts at reducing pitch have involved some form of bow or stern foil or a combination of the two. The restoring force generated by fins is speed dependent. Moreover, the pitch restoring force can be as much as five times greater than typical roll restoring forces, so pitch stabilization foils must be bigger and more rugged than conventional anti-roll fins. The cost of added appendages is an associated increase in resistance.
Bow fins can be placed on the hull or fixed to a strut below the keel. Bow fins have the greatest effect on pitching because the relative motion between the ship and the wave is greatest at the bow. However, bow fins have problems with ventilation, cavitation and vibration that preclude their viability. Fixed bow fins develop very large angles of attack, which increases ship resistance and can cause vibration due to cavitation and ventilation. To effectively reduce cavitation, active bow fins must operate continuously. If strut mounted, actuation can prove difficult. Stern fins are generally mounted forward of the propeller and may be fixed or active. Stern fins have less relative velocity than bow fins and, therefore, do not reduce pitch as well. In addition, the moment arm is shorter than for bow fins, because the pitch center is generally aft of amidships, further reducing stern fin effectiveness. In experimental studies it has been found that fixed stern fins have little affect on pitch, can actually increase heave, and increase drag considerably. Active stern fins show more promise, but still result in increased cost and complexity due to added systems as well as increasing resistance considerably.
In addition to adding appendages, substantial seakeeping improvements have been made in recent years through optimizing hull form design. However, there appears to be a limit to the degree of improvement using this approach. Increasing a ship's length improves its pitch performance by increasing its rotational inertia, thus, requiring waves of longer wavelengths to excite it. Since longer waves occur less frequently in nature, operability increases. However, increasing the length of a ship is expensive in terms of weight and cost, so factors other than seakeeping generally drive a design towards the shortest possible length.
Small waterplane area twin hull (SWATH) ships are well-known as good seakeeping hullforms. However, compared with an identical-payload monohull, a SWATH is considerably heavier and usually slower for the same horsepower. Surface combatants are required to operate at comparatively high speeds, where the resistance penalties of a SWATH are perceived to be a disadvantage. Additionally, because a SWATH has a smaller longitudinal waterplane inertia than a comparable monohull, it take less moment to cause pitching motion. As a result, both bov and stern foils are required for pitch stability.
There is a need to improve seakeeping qualities of large, high speed ocean-going vessels such as surface combatants. "Better" hull designs are not sufficient. Both longer hulls and SWATH's dramatically increase costs. Added appendages increase resistance and add design complexity resulting in increased costs over the life of the ship. Consequently, there is a long felt but unsolved need for an economical system for stabilizing pitch motions for existing and future monohull combatant and other high speed ship designs. More specifically, there is a need to stabilize both roll and pitch without adding additional systems, such as bow or stern foils, and without drastically increasing the cost of the ship.